At temperatures and pressures below its critical point (approximately 374.degree. C.), water is a poor solvent for non-polar materials (including many organic materials) and a good solvent for polar materials (including many inorganic materials). However, at and above the critical temperature of water, many organic compounds become readily soluble in water and many inorganic compounds become insoluble. For example, the solubility of inorganic salts in supercritical water is generally between 1 ppb and 100 ppm above about 450.degree. C. (U.S. Pat. No. 4,338,199 (Model No. 1), column 3, lines 39-41).
The insolubility in water of inorganic compounds at critical conditions has been a major impediment to the development of supercritical water oxidation ("SCWO") reactors.
SCWO reactors are designed to oxidize organic compounds in water at temperatures and pressures above its' critical. Under such conditions SCWO reactors are capable of effecting substantially complete oxidation (and hence destruction) of many organic materials, including many toxic organic compounds. The products of such combustion are primarily superheated water, carbon dioxide, inorganic salts and heat. For this reason, SCWO has been proposed as a method of disposing of a wide range of wastes, which contain toxic or noxious organic components. Such wastes include sewage, animal wastes, paper mill wastes and petrochemical wastes. The noxious and toxic compounds suitable for treatment include virtually all oxidizable organic compounds, including dioxins. Supercritical reactors are known and have been described in U.S. Pat. No. 2,944,396 (Barton); No. 4,292,953 (Dickinson); No. 4,543,190 (Model No. 2); and others.
Dickinson discloses that a possible limitation to the use of supercritical water oxidation exists in the amount and nature of salts dissolved in the aqueous feed to the reactor. Due to the nature of the supercritical oxidation process, such salts can become concentrated or supersaturated in the reactor. Dickinson further states that with certain types of salts, this concentration effect can result in scaling in the reactor or scaling and/or plugging in downstream heat exchange equipment (column 6, lines 33-47).
Similarly, Model states, at column 8, lines 8-34, that in conventional apparatus, inorganics tend to build up on the walls causing hot spots with subsequent destruction of the walls. To overcome this problem, Model suggests that the inner wall of the reactor be clad with corrosion resistant alloys, such as Hastelloy C, and when high concentrations of inorganic constituents are present, a fluidized bed reactor can be used. However, to do so would greatly increase the cost of the supercritical oxidation reactor.
Several different approaches have been developed to try to overcome this scaling problem. U.S. Pat. No. 4,822,497 (Hong et al.) discloses a reactor for supercritical water oxidation of organic materials having an upper supercritical zone and a lower subcritical zone. Oxidation of organic materials and inorganic materials, including salts and salt precursors, occurs in the upper zone and salts and other insoluble inorganic precipitates from the oxidation reaction are transferred to the lower subcritical zone where they redissolve and are removed from the reactor as a solution or slurry.
U.S. Pat. No. 5,100,560 (Huang) discloses a supercritical oxidation reactor having an upper supercritical temperature zone and a lower reduced temperature zone. The walls of the reactor are scraped to remove precipitates which deposit on the walls bounding the supercritical temperature zone.
However, none of these approaches has achieved any commercial success and scaling from the precipitation of inorganic salts remains a major obstacle to the use of SCWO for the treatment of organics in waste streams when the waste stream includes inorganic compounds.